Contrast agents can be introduced into a patient to enhance ultrasonic diagnosis, specifically of the vascular system. After injection, the contrast agent travels in the bloodstream from the injection site to an artery. From the artery, the contrast agent infuses into a tissue as blood flows into the capillaries of the tissue. Because contrast agents strongly interact with ultrasonic waves, they return echoes that are clearly distinguished from those returned by blood and tissue. These distinct echoes can be used to measure the rate of perfusion into a tissue to evaluate the viability of blood flow into the tissue.
Currently, qualitative images can be generated by calculating the contrast agent concentration in various mixing volumes as contrast-agent-containing or contrast-agent-free blood flows into or out of a network of mixing volumes. This method is described in Kaul, "Quantification of Myocardial Perfusion with Contrast Echocardiography," American Journal of Cardiac Imaging, Vol. 5, No. 3, Sep. 1991, Pages 200-216. Because a clear mixing volume or a set of mixing volumes within the tissue cannot be adequately defined, only qualitative comparisons can be made of the relative values of perfusion between two or more regions of tissue or between one region of tissue at two or more points in time.
Other methods generate a qualitative image by destroying contrast agents in a tissue over a much larger region than the region that is used to observe reperfusion. Because the destruction region is different from the observation region, blood containing contrast agents travels for some period of time before reaching the observation region. Because different flow patterns of blood carry contrast agents into the observation region at different times, these methods typically produce inaccurate qualitative images.
There is a need, therefore, for an ultrasound method and system for measuring perfusion that overcome the problems described above.